Van der Waals complexes of iodine with hydrogen and deuterium: Intermolecular potentials and laser-induced photodissociation studies

Abstract

A supersonic free jet expansion has been used to produce the van der Waals complexes of I2 with ortho- and para-H2 and D2. The results of fluorescence excitation and dispersed emission spectroscopy were combined to deduce the parameters of the isotropic part of the intermolecular potential. Using a Morse potential these were found to be De=122 cm−1 and ωe=106 cm−1. Observations were made which also indicate the presence of anisotropy in the potential: different absorption frequencies for complexes containing ortho and para forms of H2 and D2, and different product state distributions when ortho and para H2I2 vibrationally predissociate. Approximate measurements of vibrational predissociation lifetimes of I2H2 and some I2–rare gas complexes are reported here, and their dependence on various molecular and intermolecular potential parameters is discussed in light of current theoretical treatments, namely,the energy gap law of Beswick and Jortner and the momentum gap law of Ewing. In the case of o-H2I2, the relatively high van der Waals stretching frequency (ωe?100 cm−1) is well-matched to that of the I2 stretching frequency (ωe?128 cm−1 in the B 3Π state), resulting in good coupling between the initial bound state (prepared by laser excitation) and the final dissociative state. Hence, a comparatively short vibrational predissociation lifetime of ∼18 psec is observed.